Is it possible to paint only on selected vertices in vertex paint?
Face selection masking is available in the 3D header when in 'Vertex Paint' mode:



Because this works on faces you will only be able to paint vertices that make up a face, individual vertices or edges won't be filled.


Select the vertices you want to paint on in edit mode.
Change to 'Vertex Paint' mode.
Enable 'Face selection masking' in the 3D header. There will now be a dotted line around the selected faces.
Press Shift+K to fill the selection with the desired colours.